Highly available database clusters that move client connections between hosts

ABSTRACT

Aspects of embodiments of the present disclosure include a highly available database cluster that can maintain a connection with potentially geographically remote client application programs, including non-fault tolerant application programs, even in the event of one of the database management systems (DBMS) of the cluster becoming unavailable. For example, the database cluster can advantageously move a client connection between a failing, unbalanced, or overloaded DBMS, to another DBMS within the cluster. The database cluster can include connection managers that monitor a connection between a client application program and a primary DBMS. When one connection manager determines that the primary DBMS is unavailable, the connection manager of a secondary DBMS can assume the connection to the client application as if it were the primary DBMS. The connection manager can finish all open transactions, thereby avoiding the need to roll back the same. Moreover, the connection managers can monitor the connection at the DBMS communication level, such as, for example, the SQL*Net level.

REFERENCE TO RELATED APPLICATION

[0001] The present application claims priority benefit under 35 U.S.C. §119(e) from U.S. Provisional Application No. 60/266,908, filed Feb. 6, 2001, entitled “HIGHLY AVAILABLE DATABASE CLUSTERS,” which is incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to the field of highly available database clusters. More specifically, the invention relates to database clusters that transparently move client connections between hosts.

BACKGROUND OF THE INVENTION

[0003] A database is generally considered to be a collection of information or data organized in a way that computer programs can quickly access or select desired portions of the collection. A database management system (DBMS) includes the collection of computer programs that enable the quick storage, selection, modification, and extraction of desired portions of data from the database. Exemplary DBMSs include those commercially available from Oracle Corporation, IBM, or the like. Application programs, on the other hand, typically include client programs that connect to a DBMS to provide users the ability to interact with the data of the database, such as, for example, to select, modify, organize, delete, or the like, some or all of the foregoing data. Exemplary application programs include payroll or inventory programs, online stores, or the like.

[0004] Often, the application programs are designed to be continually connected to a DBMS, thereby having substantially continuous access to data stored within the same. Unless specifically coded to recover, these application programs typically fail when their connection to the DBMS fails or is otherwise unavailable, such as during a system failure. For many application program environments, this failure is undesirable.

[0005] System designers have created various solutions to reduce the effects of an application program losing a connection to a DBMS. For example, system designers often employ database clusters to offer backup solutions to failed systems. Database clusters can include two or more DBMSs accessing shared data files. For example, the shared data files can include data files having the same set of data from the replication of changes from one DBMS to another. Also, the shared data files can include multiple DBMSs that access the same physical storage. Through the shared data files, system designers allow one DBMS to replace another in the event of a failure.

[0006] There are several drawbacks associated with the foregoing database clustering solution, especially when employed in environments allowing for little or no down time, such as, for example, high availability solutions. For example, when a DBMS fails, the connection from the application program to the DBMS can be lost, thereby potentially losing all open transactions from the same. Additionally, data not replicated from a failing DBMS can be lost. Moreover, during load balancing, simultaneous updates of the same data on different DBMSs can occur in some replication solutions. Also, a large amount of communication traffic among a cluster, and/or hardware limitations of the same, can reduce the cost effectiveness of geographically diverse systems. Moreover, as discussed, the failure of an individual DBMS results in a failure of non-fault tolerant program applications.

[0007] On the other hand, system designers may also employ application servers in order to reduce the effects of losing a connection to a DBMS. For example, system designers often have application programs connect to an application server, where the application server includes the functionality to recover lost client connections to one or more secondary DBMSs within a database cluster. However, the application server generally includes a proprietary protocol used in communications from the application program to the application server. The proprietary protocol is generally not native to the DBMS and therefore, each connecting application program will first be routed through the application server. Thus, the application server solution is not well suited for geographically diverse storage systems.

[0008] Embodiments of the present invention seek to overcome some or all of these and other problems.

SUMMARY OF THE INVENTION

[0009] Therefore, a need exists for a database cluster that can maintain a connection with potentially geographically remote client application programs, including non-fault tolerant application programs, even in the event of a failure or other unavailability of the primary DBMS. Accordingly, aspects of embodiments of the present disclosure include a highly available database cluster that can maintain a connection with potentially geographically remote client application programs, including non-fault tolerant application programs. For example, the database cluster can advantageously move a client connection between a failing, unbalanced ,or overloaded DBMS, to another DBMS within the database cluster.

[0010] According to one embodiment, the database cluster includes connection managers which monitor a connection between a client application program and a primary DBMS. When one connection manager determines that the primary DBMS is unavailable, has an unbalanced share of the workload of the cluster, or the like, the connection manager of a secondary DBMS can assume the connection to the client application as if it were the primary DBMS. For example, the connection manager can finish all open transactions, thus avoiding the need to roll back the same. Embodiments of the connection managers can also monitor the connection at the DBMS communication level, such as, for example, the SQL*Net level. According to one embodiment, the connection managers capture enough information about the connection to restore the connection to its current state on another DBMS in the cluster.

[0011] Based on the foregoing, an aspect of an embodiment of the invention includes a data processing system comprising a database cluster which can move a connection between a remote client and a first DBMS within the cluster to a second DBMS within the cluster when the database cluster determines that the first DBMS has failed, wherein the movement of the connection is transparent to the remote client and the connection includes communication in a protocol native to the first and second DBMSs, such as, for example, SQL*Net.

[0012] Another aspect of an embodiment of the invention includes a data processing system comprising a database cluster which can move a connection between a remote client and a first DBMS to a second DBMS when the database cluster determines that the first DBMS is executing an unbalanced portion of the cluster workload, wherein the movement of the connection is transparent to the remote client.

[0013] Another aspect of an embodiment of the invention includes a method of moving a client connection from a first DBMS to a second DBMS. The method comprises monitoring a state of a client connection to a first DBMS, wherein the client connection includes communication in a protocol native to the first DBMS. The method also comprises detecting a condition of the connection which indicates the connection should be moved, and moving the client connection to a second DBMS without the client dropping the client connection.

[0014] For purposes of summarizing the invention, certain aspects, advantages and novel features of the invention have been described herein. Of course, it is to be understood that not necessarily all such aspects, advantages or features will be embodied in any particular embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] A general architecture that implements the various features of the invention will now be described with reference to the drawings. The drawings and the associated descriptions are provided to illustrate embodiments of the invention and not to limit the scope of the invention. Throughout the drawings, reference numbers are re-used to indicate correspondence between referenced elements. In addition, the first digit of each reference number indicates the figure in which the element first appears.

[0016]FIG. 1 illustrates a block diagram of an exemplary data processing system including a database cluster according to embodiments of the invention.

[0017]FIG. 2 illustrates a block diagram of exemplary connection managers of the database cluster of FIG. 1, according to embodiments of the invention.

[0018]FIG. 3 illustrates a flow chart of a fail-over process, according to embodiments of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0019] Aspects of embodiments of the present disclosure include a highly available database cluster that can move connections with one or more client program applications from a first host to a second in the event of, for example, a failure of the first host, an unbalanced or overloaded workload present on the first host, or the like. Additionally, the cluster can provide communication in the native protocol of underlying database management systems (DBMSs), thereby providing fault tolerant connections for potentially geographically remote and potentially non-fault tolerant client application programs.

[0020] According to one embodiment, the database cluster includes connection managers that monitor a connection between a client application program and a primary DBMS. When one connection manager determines that the primary DBMS is unavailable, has an unbalanced share of the workload of the cluster, or the like, the connection manager of a secondary DBMS can assume the connection to the client application as if it were the primary DBMS. In one embodiment, the assumption of the connection by the secondary connection manager is transparent to the client. Moreover, the secondary connection manager can replay or finish all open transactions, thus picking up the connection to the client in a state exactly where the primary DBMS failed. Embodiments of the connection managers can also monitor a connection at the DBMS communication level, such as, for example, the SQL*Net level. According to one embodiment, the connection managers capture enough information about the connection to restore the connection to its current state on another DBMS in the cluster.

[0021] According to one embodiment, the connection manager monitors a current state of TCP and IP protocols of a TCP/IP connection. When one connection manager determines that it should assume the TCP/IP connection, the connection manager continues the TCP conversation that the client originally started with the other connection manager.

[0022] The foregoing solution advantageously provides a database cluster offering high availability to its connecting clients, including non-fault tolerant clients, by moving connections between DBMSs within a database cluster.

[0023] To facilitate a complete understanding of the invention, the remainder of the detailed description describes the invention with reference to the drawings, wherein like reference numbers are referenced with like numerals throughout.

[0024]FIG. 1 illustrates a block diagram of an exemplary data processing system 100, according to an embodiment of the invention. As shown in FIG. 1, the data processing system 100 includes a client application program 105 (client 105) communicating with a highly available database cluster 110 (cluster 110) through a communication network 115. According to one embodiment, the client 105 comprises computer programs such as payroll or inventory programs, online stores, human resource applications, or the like, executing on one or more remote computer devices or systems.

[0025] An artisan will recognize from the disclosure herein that the client 105 can comprise virtually any client program designed to connect with a DBMS to interact with data stored therein, such as, for example, to select, modify, organize, delete, index, or the like, some or all of the foregoing data. Moreover, the artisan will recognize from the disclosure herein that the client 105 can execute on a wide variety of computer devices, such as, for example, personal digital assistants, mobile telephones, handheld computer devices, laptop computers, workstations, mainframe computers, combinations of the same, or the like.

[0026] The cluster 110 can comprise two or more DBMSs, able to access portions of shared, replicated, or otherwise mirrored data. Exemplary DBMSs include those commercially available from Oracle Corporation, IBM, or the like. According to one embodiment, the DBMSs of the cluster 110 execute on one or more hosts or other computing devices.

[0027] The communication network 115 comprises some or all of the Internet. However, an artisan will recognize from the disclosure herein that the communications network 115 can include a wide range of interactive communications mediums. For example, the communications network 115 can include interactive television networks, telephone networks, wireless data transmission systems, two-way cable systems, customized computer networks, interactive kiosk networks, automatic teller machine networks, direct links, private local or wide area networks, and the like.

[0028] In one embodiment, the client 105 connects to the cluster 110 through the communication network 115. The client 105 issues instructions or transactions including one or more operational statements to be carried out against data stored in data files accessible by the cluster 110. When the cluster 110 has executed the instructions or transactions, the cluster 110 returns an indication of the same to the client 105. Moreover, the cluster 110 can move the foregoing connection with the client 105 from a first host to a second in the event of, for example, a failure of the first host, an unbalanced or overloaded workload present on the first host, or the like. Additionally, the cluster 110 can provide communication in the native protocol of the underlying two or more DBMSs, thereby providing fault tolerant connections for the potentially geographically remote and potentially non-fault tolerant client 105. For example, the cluster 110 can monitor a connection at the DBMS communication level, such as, for example, a SQL*Net level. The cluster 110 can capture enough information about the connection to restore the connection to its current state on another DBMS in the cluster.

[0029]FIG. 1 also shows the cluster 110 including a routing device 120 communicating with a primary host 125 (Host A 125) to execute transactions against one or more shared data files 130. Additionally, FIG. 1 shows the routing device 120 having the ability to communicate with a secondary host 135 (Host B 135), which in turn also includes the ability to execute transactions against the one or more shared data files 130. According to one embodiment, Host A 125 includes a primary connection manager 140 and a primary DBMS 145, while Host B 135 includes a secondary connection manager 150 and a secondary DBMS 155. FIG. 1 also shows the primary connection manager 140 communicating with the secondary connection manager 150.

[0030] Routing device 120 comprises a device, such as, for example, a router, hub, or the like, that connects any number of computing systems or networks. Generally, routing device 120 uses information in data packets, along with a forwarding table to determine where the data packets go. According to one embodiment, the routing device 120 is configured in such as fashion as to forward all packets destined for the database cluster 110 to both the primary connection manager 140 and the secondary connection manager 150. An artisan will recognize that the function of such routing will be to enable a virtual IP address (VIP) that may be shared between hosts.

[0031] In one embodiment, the routing device 120 sends all data packets from the client 120 to both the primary connection manager 140 and the secondary connection manager 150. The secondary connection manager monitors statistics related to, for example, the number of clients connected to the primary connection manger. The primary connection manager assumes responsibility for the data packets send from the client 105 to the primary DBMS 145. Thus, when the client 105 sends transactions, in the form of data packets, through the communication network 115 to the primary DBMS 145, the data packets are routed to the primary connection manager 140, forming a connection between the primary connection manager 140 and the client 105. The primary connection manager 140 then forwards the data packets to the primary DBMS 145, forwards a copy of the data packets to the secondary connection manager 150, and monitors statistics related to, for example, the number of connected clients and the status of the secondary connection manager 150. Meanwhile, the secondary connection manager 150 receives the copied data packets, holds them in memory, and monitors statistics related to, for example, the number of connected clients and the status of the primary connection manager 140.

[0032] The primary DBMS 145 receives the data packets from the primary connection manager 140, assembles them into operational statements of transactions, and executes the same against the data files 130. The primary DBMS 145 then returns the requested data and/or acknowledgment of the received data packets back to the primary connection manager 140, which in turns forwards a copy to the secondary connection manager 150 and a copy to the respective client 105 through the communication network 115.

[0033] In an embodiment, the secondary connection manager 150 can detect a condition of the connection between the primary connection manager 140 and the client 105 from the statistics being monitored. For example, the secondary connection manager 150 can detect a failure of the connection, an unbalanced or overloaded workload on the primary connection, or the like. In such circumstances, the secondary connection manager assumes control of the connection and replays any rolled back transactions against the data files 130 through the secondary DBMS 155 as follows.

[0034] The secondary connection manager 150 communicates with the routing device 120 to acknowledge TCP requests from the client 105 to the primary connection manager 140. These acknowledgements advantageously keep the client TCP connection from timing out and failing. Additionally, the secondary connection manager 150 replays any operational statements of transactions rolled back due to, for example, the failure of the primary connection. As is generally known in the art, upon failure of a DBMS, all operational statements of open transactions (for, example, non-committed transactions) executed against the data files 130 are rolled back as if they never occurred. However, because the operational statements of open transactions are stored in the foregoing memory of the secondary connection manager 150, these operational statements from open transactions can be reexecuted against the data files 130 through the secondary DBMS 155. After replaying the foregoing operational statements, the secondary connection manager 150 begins forwarding data packets from the client 105 to the secondary DBMS 155 to be executed against the data files 130.

[0035] Based on the foregoing disclosure, the database cluster 110 advantageously moves a connection between the primary DBMS 145 and the client 105 to the secondary DBMS 155 in the cluster 110, when the primary DBMS 145 fails, becomes unbalanced, overloaded, or the like. Additionally, the database cluster 110 advantageously replays any rolled back statements of open transactions during fail-over to the secondary DBMS 155, thereby providing an assumption of the connection that is transparent to the client 105. Accordingly, the cluster 110 avoids failure of non-fault tolerant clients by moving the connection rather than allowing it to fail. Additionally, the cluster 110 advantageously provides communication in the native protocol of the underlying two or more DBMSs, thereby providing fault tolerant connections for the potentially geographically remote and potentially non-fault tolerant client 105.

[0036]FIG. 2 illustrates a block diagram of embodiments of the primary and secondary connection managers, 140 and 150, of the cluster 110, according to embodiments of the invention. FIG. 2 shows the primary connection manager 140 including a primary connection 205 communicating with a memory 210 including statistics 215, a monitor process 220 also communicating with the memory 210, and a protocol shadow 225 communicating with the memory 210 and the primary DBMS 145. Moreover, FIG. 2 shows the secondary connection manager 150 including a secondary connection 245 communicating with a memory 250 including statistics 255, a monitor process 260 also communicating with the memory 250, and a protocol shadow 265 communicating with the memory 250 and the secondary DBMS 155. In addition, the secondary connection manager 150 includes an import process 270 communicating with the primary connection 205 and a queue 275. The secondary connection manager 150 also includes a replay process 280 communicating with the queue 275 and the protocol shadow 265. Moreover, while not shown, an additional redo monitor can access one or more log files 285 associated with the primary DBMS 145. The redo monitor also can communicate with the memory 210 and review the statistics 215. FIG. 2 also shows the protocol shadow 265 accessing the one or more log files 285.

[0037] The following simplified exemplary transactions are disclosed to provide an understanding of the operation of the primary and secondary connection managers, 140 and 150 respectively, however, they are not intended to limit the scope of the disclosure. Rather, an artisan will recognize from the disclosure herein, alternative arrangements to simplify or expand one or more of the features or aspects disclosed herein.

Normal Operation

[0038] When the client 105 begins a transaction by issuing an operational statement to be applied against the data files 130, the client 105 distributes the statement across one or more data packets. The data packets are forwarded through the communication network 115 to the routing device 120, where, as disclosed, the routing device 120 forwards the packets to the primary connection 205 and to the secondary connection 245. The primary connection 205 examines statistics in the statistics 215 generated by the redo monitor. These statistics include, for example, the current location of transaction being stored in the log files 285. The primary connection transmits a copy of each data packet along with the current log file location, such as a sequence number, to the import process 270 of the secondary connection manager 150, and places a copy in the memory 210. The import process 270 stores the data packets in the queue 275. The protocol shadow 225 accesses the memory 210 and retrieves the data packets. The protocol shadow 225 forwards the packets to the primary DBMS 145, where the packets are assembled and the operational statement executed against the data files 130. Moreover, as is generally known in the art, the DBMS can also keep a record or log of the executed statement, generally in the log file 285.

[0039] The DBMS 145 forwards a result of the statement and/or and acknowledgement of receipt of the same, back to the protocol shadow 225, preferably in one or more acknowledgement data packets. The protocol shadow 225 transfers the data packets back to the memory 210, where they are picked up by the primary connection 205. The primary connection 205 forwards a copy of the data packets to the import process 270 and to the client 105. Thus, the client 105 receives the results and/or acknowledgement of the transmitted statement of an open transaction.

[0040] The client 105 may then desire to finalize, or commit the transaction against the data files 130. In such case, the client 105 issues a commit statement, which is forwarded to the primary DBMS 145 and the import process 270, along with the subsequent result and/or acknowledgement, in a manner similar to that disclosed. In one embodiment, the protocol shadow 225 stores sufficient data from the data packets that it can assemble the statements of a given transaction. When the protocol shadow 225 determines the data packets for a commit statement have been sent to the primary DBMS 145, the protocol shadow attaches a marker to the result/acknowledgement data packets associated with the primary DBMS 145 acknowledging execution of the commit statement. According to one embodiment, the marker comprises a location marker, such as, for example, a sequence number from the primary DBMS 145. Then, as disclosed, the result/acknowledgement data packets are transmitted with their marker to the import process 270. According to one embodiment, the import process 270 recognizes the marker placed on the data packets associated with the commit statement, and recognizes that the entire transaction has been executed by the primary DBMS 145 against the data files 130. Therefore, the import process 270 deletes the data packets associated with the now finalized transaction from the queue 275.

[0041] Based on the foregoing, the protocol shadow 225 and the import process 270 advantageously work together to ensure that only the data packets associated with open transactions remain in the queue 275.

[0042] The primary connection 205 also stores the statistics 215 related to the connection with the client 105 in the memory 210. In one embodiment, the statistics include sufficient information for the monitor process 220 to determine whether the primary connection 205 has failed, is processing an unbalanced or overloaded workload, or the like, and whether the secondary connection 245 has failed, is processing an unbalanced or overloaded workload, or the like. For example, the statistics 215 can include the number of clients seen by the primary connection 205, the number of clients seen by the secondary connection 245, the status of communication with secondary communication manager 150, or the like. The primary connection 205 acquires the statistics 215 corresponding to information from the secondary connection manager 150 through the connection between the primary connection 205 and the secondary connection 245. Moreover, according to one embodiment, the foregoing status of the secondary communication manger 150 can be ascertained through straightforward ping or ping-like commands.

Fail-Over

[0043]FIG. 3 illustrates a flow chart of a fail-over process 300, according to embodiments of the invention. As shown in FIG. 3, the fail-over process 300 begins with BLOCK 305 where the cluster 110 monitors the statistics of one or more connections with one or more clients. In the foregoing example, the monitoring corresponds to the monitor processes 220 and 260. In BLOCK 310, the cluster 110 detects the need to move the connection from one DBMS to another. For example, the monitor 260 may determine that the primary DBMS 145 has failed, become unbalanced, overloaded, or the like, and determine that the secondary connection manager 150 should assume the connection with the client 105. When the determination that a connection move is desired, the fail-over process 300 proceeds to BLOCK 320, where the cluster 110 moves the connection from one DBMS to another without losing the connection or causing even a non-fault tolerant client to fail. For example, the secondary connection 245 can communicate with the routing device 120 to assume the IP address (or VIP) of the primary DBMS 145. Additionally, the secondary connection manager 150 can replay all statements of open transactions which were rolled back in the data files 130. Accordingly, the move is transparent to the client 105 who does not lose the connection and does not know that a change has been made.

[0044] According to one embodiment, BLOCK 320 can include SUBBLOCK 321, where the cluster 110 instructs the routing device 120 to forward communication from the client to another DBMS. For example, as disclosed, the secondary connection 245 can assume the IP address of the primary DBMS 145. BLOCK 320 can also include SUBBLOCK 322, where the cluster 110 can send a keepalive message to one or more clients to ensure against failure of the connection to the same. According to one embodiment, the client 105 resends data packets which are not responded to or otherwise acknowledged by the cluster 110. When the client 105 resends the same data packets a predetermined amount of times, the client 105 may register a failure of the connection, thereby causing non-fault tolerant clients (such as those clients not programmed to recover) to also fail. Thus, during the fail-over process 300, the cluster 110 can respond to the client 105 with a message or acknowledgement that keeps the client 105 from resending the same data packets, therefore keeping the client from determining that the connection has failed. According to one embodiment, the secondary connection 245 sends the foregoing keepalive messages.

[0045] BLOCK 320 of the fail-over process 300 can also include SUBBLOCK 324 where the cluster 110 replays any statements from open transactions that were rolled back during the failure of the primary DBMS 145. For example, the replay process 280 can access the queue 275 to retrieve data packets associated with rolled back transactions and to forward them to the protocol shadow 265. For example, as disclosed in the foregoing, the import process 270 removes the statements associated with all finalized or committed transactions, thereby leaving only rolled back transactions in the queue 275.

[0046] BLOCK 320 of the fail-over process 300 can also include SUBBLOCK 326 where the cluster 110 removes any leftover committed transactions that may have slipped through. For example, it is possible that Host A 125 can fail after the primary DBMS 145 executes a commit statement for a particular transaction, but before the result/acknowledgement of the same can be transmitted to the import process 270. Thus, the secondary connection manager 150 believes the statements associated with the foregoing transaction were rolled back, e.g., because they were left in the queue 275, and therefore, the replay process 280 will forward the already committed statements to the protocol shadow 265. In one embodiment, the protocol shadow 265 parses the log file 285 of the primary DBMS 145 to ensure a commit statement associated with the open transaction was not received. When the protocol shadow 265 determines that a commit statement was received, the protocol shadow 265 deletes the statements associated therewith before their associated data packets are forwarded to the secondary DBMS 155 to be executed against the data files 130.

[0047] BLOCK 320 of the fail-over process 300 can also include SUBBLOCK 328 where the cluster 110 establishes communication between the client and the secondary DBMS. For example, after all rolled back statements are either executed against the data files 130 through the secondary DBMS 155 or deleted from the queue 275 by the protocol shadow 265, the protocol shadow 265 begins accessing new data packets stored in the memory 250 by the secondary connection 245 after it assumed the connection to the client 105 from the primary connection manager 140. Thus, after bringing the secondary DBMS 155 back up to the point of failure of the primary DBMS 145, the secondary connection manager 150 performs operations similar to the normal operations of the primary connection manager 140 as disclosed above.

[0048] According to one embodiment, the system administrator of the database cluster 110 can designate whether the secondary connection manager 150 through the monitor process 260 fails-back to the primary connection manager 140 after the cause of failure of the same is repaired, or whether the secondary connection manager 150 simply becomes the primary and vice versa.

[0049] Although the foregoing invention has been described in terms of certain preferred embodiments, other embodiments will be apparent to those of ordinary skill in the art from the disclosure herein. For example, the data packets captured from the primary connection manager 140 can be replicated to other DBMSs by replaying the same on the other DBMSs. This replication has several advantages over other replication techniques including a potential reduction in the traffic keeping the database cluster synchronized, thereby advantageously providing economical replication of geographically diverse data files.

[0050] The captured data packets can also be used to assist a transaction log based replication system. For example, the data packets can be directed to the other databases in the cluster prior to committing the transactions. Accordingly, committed transactions on a particular DBMS are not lost when the DBMS fails, as these transactions may advantageously be replayed on the other DBMSs in the cluster.

[0051] The captured data packets can also be used to assist a transaction log based replication system when posting replicated modifications. Some modifications (such as a vertical table update or DDL operation) may be difficult to replicate via a log-based replication. When the original data packets are available, posting the original SQL rather than the data from the transaction log may be more efficient and straightforward.

[0052] According to another embodiment, software may be added just below the client 105, thereby providing a mechanism to replay incomplete transactions. For example, a typical client application does not access the database directly, but instead uses some type of intermediate layer such as ODBC or JDBC, OCI, or the like. The foregoing added software can advantageously replace this intermediate layer.

[0053] Additionally, other combinations, omissions, substitutions and modifications will be apparent to the skilled artisan in view of the disclosure herein. Accordingly, the present invention is not intended to be limited by the reaction of the preferred embodiments, but is to be defined by reference to the appended claims.

[0054] Additionally, all publications, patents, and patent applications mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication, patent, or patent application was specifically and individually indicated to be incorporated by reference. 

What is claimed is:
 1. A database cluster which avoids client failure by connecting to multiple nodes of the cluster, the database cluster comprising: a first computing system including: a primary connection manager which forms a client connection with and receives transactions from at least one client, and a primary database management system (DBMS) which communicates with the primary connection manager to receive the transactions and executes the transactions on data stored in one or more data files; and a second computing system including: a secondary connection manager, and a secondary DBMS which communicates with the secondary connection manager and can access data stored in the one or more data files, wherein when the second connection manager determines that a predetermined condition is met, the second connection manager receives data from the client connection, replays incomplete portions of open transactions on the data through the secondary DBMS, and begins to receive additional transactions from the at least one client to be executed against the one or more data files.
 2. The highly available database cluster of claim 1, wherein the predetermined condition comprises a failure of the first computing system.
 3. The highly available database cluster of claim 1, wherein the predetermined condition comprises a failure of the primary DBMS.
 4. The highly available database cluster of claim 1, wherein the predetermined condition comprises an unbalanced workload between the first and second computing systems.
 5. The highly available database cluster of claim 1, wherein the primary connection manager and the secondary connection manager communicate with one another.
 6. The highly available database cluster of claim 5, wherein the primary connection manager transmits copies to the secondary connection manager of data packets which include the transactions and responses or acknowledgements to the transactions.
 7. The highly available database cluster of claim 5, wherein the primary connection manager and the secondary connection manager exchange statistics in order to monitor the client connection.
 8. The highly available database cluster of claim 7, wherein the statistics include the number of clients connected to the primary connection manager.
 9. The highly available database cluster of claim 7, wherein the statistics include the number of clients the secondary connection manager can see connected to the primary connection manager.
 10. The highly available database cluster of claim 7, wherein the statistics include whether the secondary connection manager can communicate with the primary connection manager.
 11. A primary and at least one secondary connection manager of a database cluster, which manage a connection between at least one client and two or more database management systems (DBMSs), wherein the primary and at least one secondary connection manager can move the connection from the primary connection manager to the at least one secondary connection manager while providing protocols for the connection native to the two or more DBMSs, the primary and secondary connection manager comprising: a first memory; a primary connection configured to form a connection with a client and to place statements from transactions from the client into the first memory; a primary protocol shadow configured to retrieve the statements and forward the statements to a primary DBMS; a secondary memory; a secondary connection configured to receive transactions from the connection with the client when one or more predetermined conditions are met and to place new statements from the transactions from the client into the second memory; at least one process configured to replay any incomplete statements of open transactions; and a secondary protocol shadow configured to connect to the at least one process until the incomplete statements are forwarded to a secondary DBMS and then to connect to the secondary memory to retrieve the new statements and forward the new statements to the secondary DBMS.
 12. The primary and at least one secondary connection manager of claim 11, wherein the protocol native to the two or more DBMSs comprises SQL*Net.
 13. The primary and at least one secondary connection manager of claim 11, wherein the at least one process further comprises: an import process configured to retrieve the statements from the primary connection and store those statements associated with open transactions; and a replay process configured to access the stored statements and to forward the stored statements to the secondary protocol shadow.
 14. The primary and at least one secondary connection manager of claim 11, wherein the secondary protocol shadow is configured to access a log file of the primary DBMS to ensure against replaying of statements of closed transactions.
 15. The primary and at least one secondary connection manager of claim 11, wherein the primary and secondary connections communicate with one another.
 16. The primary and at least one secondary connection manager of claim 15, wherein the primary connection and the secondary connection exchange statistics in order to monitor the connection.
 17. A method of providing native protocol access and transparent fail-over to a client connection thereby avoiding a client failure when a primary host fails, the method comprising: rerouting a client connection between a first host and a client to a second host; replaying at least one statement from open transactions, wherein the at least one statement includes a statement received but not committed by the first host when the client connection was moved from the first host; and establishing communication between the second host and the client over the client connection.
 18. The method of claim 17, further comprising sending keepalive messages to the client in order to keep the client from dropping the client connection.
 19. The method of claim 17, wherein the replaying at least one statement further comprising removing leftover statements of closed transactions.
 20. A method of providing transparent fail-over to a client connection thereby avoiding a client failure when a primary database management system DBMS fails, the method comprising: monitoring statistics of a client connection between a first DBMS and a client; determining from the statistics a need to move the client connection to a second DBMS while keeping the client connection alive from a perspective of the client; rerouting the client connection to the second DBMS; replaying any statements from open transactions rolled back when the client connection was moved from the first DBMS; and establishing communication between the second DBMS and the client over the client connection.
 21. A data processing system which provides transparent fail-over to a client connection, thereby avoiding a client failure when a primary host fails, the data processing system comprising: a first host configured to accept a client connection from a client; a connection manager which reroutes the client connection to a second host without recognition by the client; and a replay process which forwards to the second host at least one incomplete statement from open transactions when the client connection was moved from the first host, wherein the connection manager establishes communication between the second host and the client over the client connection.
 22. The data processing system of claim 21, wherein the client communication comprises a protocol native to the primary host.
 23. The data processing system of claim 22, wherein the protocol comprises SQL*Net. 